1. Field of the Invention
This invention relates to powder coating compositions which are solid at temperatures below 30.degree. C. and liquid above 150.degree. C. and which are based on A) copolymers containing carboxylic acid anhydride groups and epoxide groups and B) polyhydroxyl compounds.
2. Description of the Prior Art
It is known to use compositions containing polyacrylates having at least two carboxyl groups per molecule and polyepoxide compounds, optionally based on acrylates, as powder lacquers. Japanese Patent Application 73-9,319 (Publication number: 49-116,134; Chem. Abs. 82(24):157882e) describes heat curable powder coating compositions containing A) a solid acrylate resin prepared by the reaction of a hydroxyl group-containing copolymer with a polycarboxylic acid or its anhydride and B) an epoxide group-containing copolymer. The two polymers are mixed together as melts at temperatures of about 100.degree. C. and the mixtures are ground up after cooling. The coatings may be applied to metal surfaces and hardened at temperatures of about 200.degree. C.
DE-OS 23 47 680 describes heat curable powder lacquers containing A) 50 to 97% of an epoxide group-containing copolymer having a molecular weight of 3,000 to 15,000 and a softening point of 80.degree. to 150.degree. C. and B) 3 to 50% of a carboxyl group-containing hardener having a softening point of at least 50.degree. C., wherein the hardener may be a copolymer containing carboxyl groups.
Japanese Patent Application 75-146,705 (Publication Number: 52-069,936; Chem. Abs. 88(12):73374a) describes heat curable powder lacquers based on glycidyl (meth)acrylate copolymers containing A) 100 parts of a copolymer (T 30.degree. to 100.degree. C., molecular weight 3,000 to 70,000) containing 10 to 50% by weight of glycidyl(meth)acrylate, 30 to 85% by weight of acrylic acid C.sub.1 -C.sub.12 -alkyl esters and 0 to 60% by weight of another comonomer, B) 0.1 to 30 parts of a copolymer which regulates flow properties (T.sub.g &gt;50.degree. C., molecular weight &gt;1000), containing 30 to 85% by weight of (meth)acrylic acid alkyl esters, 1 to 50% by weight of a monomer having a glycidyl, carboxyl, hydroxyl or amide group and 0 to 69% by weight of another monomer and C) 3 to 55 parts of an aliphatic dicarboxylic acid having 4 to 20 carbon atoms. The lacquers adhere firmly to metals without previous surface treatment.
U.S. patent Ser. No. 3,991,132 and U.S. patent Ser. No. 3,991,133 describe heat curable powder lacquers obtained from A) an epoxide-containing copolymer containing 5 to 20% of glycidyl methacrylate and 80 to 95% of other comonomers (T.sub.g 40.degree. to 90.degree. C., molecular weight 1500 to 15,000) and B) an anhydride cross-linking agent, either a dicarboxylic acid anhydride or a dicarboxylic acid anhydride homopolymer, provided that the copolymer is difunctional. In U.S. patent Ser. No. 3,991,132 the copolymer is synthesized from at least 2 to 10% of an unsaturated acid amide and in U.S. patent Ser. No. 3,991,133 the copolymer is synthesized from at least 2 to 10% of a C.sub.5 -C.sub.7 - hydroxyalkyl(meth)acrylate in addition to 70 to 93% of other comonomers such as methyl or butyl methacrylate, styrene or vinyl toluene. Further, a quantity of the anhydride hardeners is replaced by a hydroxycarboxylic acid so that 0.1 to 0.4 carboxyl groups of the hydroxycarboxylic acid are present for every functional group of the copolymer.
It is also known to use components containing anhydride groups with components containing hydroxyl groups as powder lacquers. Lacquer systems of this type are described in U.S. Pat. Nos. 4,069,275 and 4,101,606, British Patents Nos. 1 366 081 and 1 561 828 and European Patent Applications Nos. 73 022 and 209 377. Such powder lacquers are normally hardened at temperatures of 130.degree. to 200.degree. C., but it has been found that the cross-linking reactions do not proceed to completion at relatively low temperatures, e.g., 130.degree. to 160.degree. C., so that the solvent resistance of the lacquer films obtained is not sufficient. it is only at hardening temperatures of 170.degree. to 200.degree. C. that lacquer films with acceptable solvent resistance are obtained.
In none of the previously described publications are copolymers containing both cyclic carboxylic acid anhydride groups and epoxide groups described as binders or powder lacquers.
It has now surprisingly been found that copolymers of olefinically unsaturated compounds containing both cyclic carboxylic acid anhydride groups and epoxide groups in the same macromolecule are excellent binders for powder lacquers when used in combination with polyels if they have the required softening point or range. The special advantage of such binder compositions is that they enable solvent-resistant lacquer films to be obtained at low stoving temperatures, e.g., at 130.degree. to 150.degree. C.
Another advantage lies in the fact that initially component A) does not contain free carboxyl groups which are required for cross-linking with the epoxide groups of component A). The required carboxyl groups are subsequently formed by the reaction of the carboxylic acid anhydride groups of component A) with the hydroxyl groups of component B). These new binder compositions therefore have considerably better storage stability at room temperature than comparable state of the art systems containing carboxyl groups.
Due to the high cross-linking densities which may be obtained, powder coatings having a very high level of chemical and solvent resistance can be obtained from the binder compositions according to the invention.
The binder compositions according to the invention therefore on the one hand have excellent storage stability at room temperature or at moderately elevated temperatures below their softening range and on the other hand result in coatings having excellent properties.